The present invention relates to a rubber composition which is preferably used as an automobile vibration-isolating rubber, and to an automobile vibration-isolating device having a strut mount obtained from the same.
A natural rubber composition or a rubber composition comprising a blend of a natural rubber and a polybutadiene rubber or a polystyrene-butadiene copolymer rubber has been hitherto used as an automobile vibration-isolating rubber of a strut mount, an engine mount, a body mount, a bush or the like.
In order to reduce a passenger compartment noise which is caused by road noise emanating from an automobile road surface, it is advisable that in a strut mount, for example, a dynamic spring constant in a high frequency region of from 100 to 300 Hz be decreased and a dynamic factor (dynamic-to-static modulus ratio) be closer to 1. Therefore, in the above-mentioned vibration-isolating rubber, a rubber composition comprising a natural rubber and a polybutadiene rubber has been recently often selected and used. However, since the polybutadiene rubber which has been used to date is produced in the presence of a cobalt catalyst or a nickel catalyst and has a cis content of less than 97%, an effect of a decrease in the dynamic factor is not sufficient as will be later described, and a satisfactory decrease in the dynamic factor has been in demand.
Especially, as an automobile weight is reduced to meet social demands for energy saving and exhaust gas control in recent years, there is a growing tendency that a rolling resistance of a tire is decreased or a weight of a tire is reduced. However, a tire having the low rolling resistance or the light-weight tire generally tends to increase the road noise. Thus, the decrease in the dynamic factor has been in high demand.